When is the best time to thermal desalination plant washing?

The Best Time to Flush a Thermal Desalination Plant

Is your plant’s fresh water output decreasing or your steam/energy consumption increasing? These are warning signs of scaling in your thermal desalination system.

But most importantly: When should you perform chemical flushing? Not too early, which would be costly, and not too late, which would cause irreparable damage. This article presents six key indicators and operational guidelines for determining the best time to flush a thermal desalination plant (MED/MSF).

Why is thermal desalination plant flushing timing critical?

Thermal desalination plants (such as MED and MSF) heat the graded water to deposit dissolved salts as solids (scale) on the surfaces of the heat transfer tubes. These layers, like an insulator, impede the flow of heat.

Three main factors determine the rate of scale formation:

1. Feedwater hardness (concentration of Ca²⁺, Mg²⁺, SO₄²⁻)

2. Operating temperature (the higher the temperature, the faster the scaling)

3. Presence or absence of anti-scale agents in the system

If chemical flushing is done early, unnecessary costs will be incurred. However, if it is done late:

• Under-deposit corrosion will begin
• Equipment life will be drastically reduced
• The risk of unplanned shutdowns will increase

Therefore, the best time to flush is when system performance has measurably decreased

But it is also time to prevent structural damage.

Key indicators to determine when it is time to flush

1. Decrease in fresh water flow (more than 15–20%)

Outlet flow is one of the simplest and most reliable indicators.

• 0–10% decrease: No immediate need for flushing; however, further monitoring is necessary.
• 15–25% decrease: Golden point for implementing preventive flushing.
• More than 25% decrease: The deposits are probably very hard and a combination of methods may be required.

Note: Flow should be measured under the same conditions (pressure, temperature, feed flow).

2. Increased steam or energy consumption

Scales reduce heat transfer. To compensate, the system requires more steam or energy.

• If steam consumption increases by more than 10% compared to baseline, an alarm is triggered.
• This increase translates directly into daily operating costs.

3. Change in temperature profile between effects (in MED systems)

In a healthy MED system, the temperature difference between successive effects is quite regular.

• A sudden increase in the temperature difference in an effect indicates insulation due to scale.
• This data can be extracted from DCS or SCADA systems.

4. Increased pressure in feed or steam lines

If pumps are generating more pressure to pass flow through the scale-laden pipes, this is an indication of flow restriction.

• Higher pressure = faster pump wear
• There is also a risk of complete blockage in the future.

5. Change in the quality of the outgoing fresh water

Although less common, in severe cases of scaling, local leaks in the pipes may develop, leading to an increase in the TDS of the outgoing water.

6. Maintenance history and feed water quality

• Is the anti-scale system being dosed correctly?
• Is the feed water experiencing a seasonal change (e.g. summer)?
• Was the last chemical flush more than 24 months ago?

If the answer to any of these questions is “yes”, then it is likely that a flush is needed — even if all other indicators are still neutral.

Smart maintenance cycle: Why is “time-based” flushing not enough?

Many units perform flushing on a scheduled basis every 2 years. This is simple, but not optimal.

Why? Because the operating conditions of each unit are different:

• Unit A with filtered seawater and effective anti-scale → may operate without problems for 36 months.
• Unit B with unstable feed water → may require flushing after 12 months.

Smart Maintenance = Continuous Monitoring + Data-Driven Decision Making

Steps of a Preventive Maintenance Program

1. Record key parameters (flow, energy consumption, temperature, pressure) every week

2. Collect sediment samples every 6 months (or at each planned outage)

3. Laboratory analysis of sediment to identify chemical composition

4. Compare trends with baseline conditions

5. Make targeted decisions based on predefined thresholds

How does laboratory analysis help in decision making?

Even if the operational indicators are still within limits, physical sampling of the deposit can be a strong predictor.

Common analysis methods:

• XRF (X-ray fluorescence): Determination of major elements (Ca, Mg, Si, S)
• XRD (X-ray diffraction): Identification of crystalline phases (CaCO₃vs. CaSO₄)
• SEM-EDS: Investigation of surface structure and element distribution

With this information:

• The chemical wash formulation can be precisely adjusted
• The source of the deposit can be identified (e.g., a leak in the pretreatment system)
• The next wash interval can be predicted more accurately

Common mistakes in washing scheduling

❌ Waiting until the system is completely down

Many do not take action until the system is practically down. This approach increases repair and replacement costs several times over.

❌ Using chemical wash without deposit analysis

“One size fits all” is not effective. Improper washing may harden deposits or damage metals.

❌ Ignoring historical data

If your system suffers from a drop in flow rate every 18 months, why wait until the problem recurs?

❓❓❓ Frequently Asked Questions (FAQ) ❓❓❓

❓ Can chemical flushing be performed during operation?

No. Chemical flushing requires a complete shutdown of the system and the creation of an independent circulation circuit. But it can be incorporated into scheduled shutdowns (e.g. filter changes).

❓ Can the use of anti-scalers eliminate the need for flushing?

No — it delays it. Even with the best anti-scalers, scaling continues at a very low rate. Periodic flushing is still necessary.

❓ What is the difference between MED and MSF in flushing timing?

MSF operates at a higher temperature (up to 120°C), so scaling is faster. Flushing intervals are usually shorter in MSF than in MED.

❓ Will the same problem recur after flushing?

If the root cause (such as poor feed water or lack of anti-scale) is not addressed, yes. Flushing only treats the symptoms, not the root of the problem.

Conclusion: Data-driven decision-making, not guesswork ⭐⭐⭐⭐⭐

The best time to flush a thermal desalination plant is when the data screams at you — not when the system stops breathing. By continuously monitoring performance indicators and regularly analyzing scale, you can:

• Reduce maintenance costs by 20–30%
• Extend equipment life by 5–10 years
• Prevent unexpected production line downtime

Step 1:Get a 6-month performance report of your system today.

Step 2:If you notice a downward trend, collect and analyze a scale sample. This is the easiest way to get ahead of the crisis.

If you need expert advice to identify the type of scale or the optimal selection of chemicals, Abrizan’s specialists, with over 20 years of experience in advanced laboratories, are ready to provide customized solutions to various industries.

☎️Free call and consultation with Abrizan specialists☎️